1. Field of the Invention
The present invention relates to an engine protection method and apparatus, and an engine power control method and apparatus for a cargo handling vehicle, and more particularly to an engine protection method and apparatus which can automatically prevent an engine from being damaged due to overheat of the engine in a cargo handling vehicle such as a fork lift truck or the like, or which can prevent an engine associated part from being damaged in the case where a supplied petroleum gas fuel becomes a low fuel pressure in a single point injection (SPI) type internal combustion engine having a fuel injector arranged on an upstream side of a collector portion of an intake manifold of the engine, and an engine power control method and apparatus for a cargo handling vehicle constructed to restrict a maximum speed, which control method and apparatus can control an engine rotational speed so that a shaft torque becomes maximum at the time of cargo handling.
2. Description of the Related Art
In a conventional cargo handling vehicle such as a fork lift truck, a warning light is provided in a driver's stand panel, so that in the case where an engine overheats, a warning is given by turning on the warning light. Further, there has been a cargo handling control apparatus for a fork lift truck which controls an opening of a throttle valve so that an engine is brought into an idle state when a temperature of an engine cooling water is increased to the outside of a proper temperature range for operation. In the idle state, the throttle valve is closed while a small amount of a fuel is supplied from a small hole formed on the engine side, whereby the engine is maintained in a low speed rotation state, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 4-231628.
In the prior art mentioned above, in the case of the overheat being warned by the warning light, in the case where the driver is not aware of lighting of the warning light and leaves the engine as it is, there is a possibility that the engine is damaged due to the overheat, so that it is necessary that the driver always pays attention to the warning light. Further, in the latter prior art, since the engine is held in the idle state, an air intake amount is substantially shut off, and the engine is kept in a state where the fuel is cut. Accordingly, it is possible to prevent the engine from being damaged due to the overheat, however, when the engine rapidly stops, a lot of time is required for eliminating the overheat, so that there is a possibility that the cargo handling work with time constraints is adversely affected.
Further, conventionally, there have been an SPI system in which a fuel injector is placed on an upstream side of a collector portion of an intake manifold of the engine so as to supply a fuel to each of the cylinders, and a multi point injection (MPI) system in which fuel injectors are provided in respective cylinders to supply fuel to the respective cylinders. The latter is common at the present time, however, the SPI system is used in some of engines at the present time for the reason of cost and simple structure as compared to the MPI system, and is frequently used in engines for fork lift trucks.
The fuel includes gasoline, a light oil and a liquefied petroleum gas (LPG). In the case where the liquefied petroleum gas is employed and the petroleum gas fuel is supplied to the engine by the SPI system, the supply of the petroleum gas fuel is carried out by utilizing a vapor pressure within a gas cylinder and adjusting the pressure to an injection pressure of about 0.3 kgf/cm2 by means of a vaporizer. Thus, in the case of the liquefied petroleum gas, no fuel pump is used.
The liquefied petroleum gas for an engine fuel is distributed as a mixed gas of a propane (C3H8) component and a butane (C4H10) component. A pressure of the mixed gas within a gas cylinder varies between 0.6 and 13.0 kgf/cm2 depending on a temperature and a mixture ratio thereof, and a fuel pressure tends to reduce due to properties of a vaporizer at a high flow rate.
Accordingly, if the fuel pressure is less than 0.26 kgf/cm2, it is impossible to satisfy a required amount of the gas fuel to be injected, so that a misfire, a backfire or the like due to a lean gas is generated, and that an exhaust gas temperature is increased to cause damage to a catalyst. Therefore, in the case where the fuel pressure is equal to or less than a certain level, it is necessary to carry out a rotation control and a throttle opening control so as to prevent a flow rate of the gas fuel from being insufficient.
As one example of the rotation control, Japanese Patent Application Laid-Open Publication No. 11-21054 discloses or suggests a method of controlling a fuel leaning, an engine knocking and the like at a low fuel pressure in a direct-injection engine using gasoline as a fuel, which engine is different from the engine using the liquefied petroleum gas mentioned above. Since in the direct-injection engine, fuel injection is carried out in a later stage of a compression stroke in which a pressure within a combustion chamber is very high, it is necessary to supply the fuel at a high pressure to a fuel injection valve. Accordingly, in addition to a motor-driven low pressure pump disposed within a fuel tank for pumping the fuel toward the fuel injection valve, the engine is provided with a high pressure pump which further pressurizes the fuel from the low pressure pump so as to supply the fuel to the fuel injection valve.
Since the high pressure pump is driven by a cam shaft of the direct-injection engine, the discharge amount of the high pressure fuel is increased with an increase of an engine rotational speed, whereas, since the low pressure pump is driven by a motor, the discharge amount is fixed regardless of the engine rotational speed.
Accordingly, it is necessary that the discharge amount of the low pressure pump has a sufficient margin in comparison with a maximum discharge amount of the high pressure pump at the allowable highest rotational speed of the direct-injection engine. However, if the discharge amount of the low pressure pump decreases due to a reduction in supply voltage or the like, the fuel supplied to the combustion chamber becomes lean, so that there is a possibility that an engine knocking is caused by overheat of the combustion chamber.
This can be solved by making the low pressure pump have a very large capacity, however, there will be problems of an increase in weight and electric power consumption, and the like. Accordingly, Japanese Patent Application Laid-Open Publication No. 11-21054 has proposed an apparatus which is made as compact as possible and which is constructed so as to prevent a fuel leaning and an engine knocking from being generated.
However, the conventional example mentioned above is an approach to solving the problem in the case of the direct-injection engine using gasoline as a fuel. Since the low fuel pressure problem raised in the SPI type engine using a liquefied petroleum gas is absolutely different in cause, the conventional approach is not applicable to such an SPI type engine, resulting in a change of perspective being required to solve the problem.
As a method of preventing the engine rotational speed from being increased to a certain level or more, there are measurements such as a fuel cut, an ignition cut and the like, however, there is a possibility that a lean combustion of an air-fuel mixture and an increase of a catalyst temperature are caused.
In the case of the fuel cut, since the fuel is left within an intake manifold just after the fuel cut, the combustion in a fuel cut state is not immediately carried out, but the lean combustion is induced instead.
Further, in the case where the fuel pressure of the petroleum gas decreases, the amount of the fuel gas in the same injection time decreases, so that the same phenomenon occurs. In this case, since the combustion within a catalytic converter is generated due to a mis-ignition within the engine combustion chamber, there is a possibility that the catalyst is damaged and a backfire in the intake manifold is generated due to restriction of a combustion speed.
Further, the increase of the catalyst temperature is caused by combustion within the catalyst in the case where the mis-ignition occurs due to the lean fuel, and the air-fuel mixture flows into an exhaust pipe. Also, in the case where the ignition cut is carried out, the air-fuel mixture flows into the exhaust pipe, so that there is a possibility of the same result.
In a conventional industrial vehicle such as a cargo handling vehicle or the like, an engine drives drive wheels as well as a hydraulic pump so as to carry out a cargo handling work with a cargo handling cylinder actuated by a pressurized fluid discharged from the hydraulic pump. The cargo handling vehicle is provided with an accelerator pedal so that a traveling speed is changed in correspondence to a depressing angle of the accelerator pedal or an accelerator pedal angle. Further, the cargo handling vehicle is provided with a maximum vehicle speed control apparatus for controlling a maximum rotational speed of the engine so as to control a maximum vehicle speed, for the purpose of accident prevention during traveling on the premises or yard of the plant. For example, in order to prevent the vehicle speed corresponding to a target rotational speed of the engine from exceeding the maximum vehicle speed, the maximum vehicle speed control apparatus is constructed in such a manner that a throttle opening is fixed even when the depressing amount of the accelerator pedal is made large, whereby the rotational speed of the engine is controlled to restrict the vehicle speed.
Further, a certain type of cargo handling vehicle, as disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2001-278599, is provided with an optical sensor for detecting an illuminance in the cargo handling yard, to thereby reduce the traveling speed in the case where the cargo handling yard is dark. For example, as shown in FIG. 11A, optical sensors 55 are mounted to an upper surface of a head guard 52 of a cargo handling vehicle 51, a top and both side surfaces of masts 53 and both side portions of a driver's stand panel 54, respectively, whereby the vehicle speed is controlled by comprehending the illuminance in the environment by means of a control apparatus on the basis of an amount of light detected by the optical sensors 55. As shown in FIG. 11B, when the control apparatus of the cargo handling vehicle 51 determines the illuminance in the environment, wherein the illuminance is more than a predetermined threshold value TH, the vehicle speed is not limited and permitted to reach a limit speed M, whereas, when the illuminance in the environment is equal to or less than the predetermined threshold value TH, the maximum vehicle speed is limited in proportion to the illuminance. For example, when the illuminance in the environment is S, the maximum vehicle speed is limited to N.
However, in the conventional cargo handling vehicle, since the drive wheels are driven by the engine and the cargo handling cylinder is actuated by the pressurized fluid discharged from the hydraulic pump driven by the engine, the maximum rotational speed of the engine is limited in the case where the cargo handling work yard is dark, with the result that a torque required for the cargo handling work can not be obtained, to thereby pose a problem for the cargo handling work.